Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen (or a gas containing hydrogen) is supplied to an anode side of the fuel cell and oxygen is supplied as an oxidant to a cathode side. The oxygen can be either a pure oxygen (O2) or air. PEM fuel cells include a membrane electrode assembly (MEA) having a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. If a diffusion medium (DM) and/or a barrier layer is bonded to the MEA and optionally sealed with a gasket as a unit, the unit is known as a unitized electrode assembly (UEA). To form a single fuel cell, a MEA or a UEA is disposed between two bipolar plate assemblies.
Bipolar plate assemblies include an anode plate and a cathode plate for adjacent fuel cells in the fuel cell stack. Flow channels are provided on the anode side of the bipolar plate assemblies to facilitate the flow of hydrogen to the anode side of each MEA. Flow channels are provided in the cathode side of the bipolar plate assemblies to facilitate the flow of oxygen to the cathode side of the MEA or UEA. The bipolar plate assemblies are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells. Additionally, the individual anode and cathode plates forming bipolar fuel cell plate assemblies typically define coolant flow channels therebetween to facilitate the flow of a cooling fluid therethrough to cool the fuel cell.
A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack may include a UEA, and each UEA provides an increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. A typical arrangement of multiple cells in a stack is shown and described in commonly owned U.S. Pat. No. 5,763,113, hereby incorporated herein by reference in its entirety. The fuel cell stack receives a cathode input gas such as oxygen, typically a flow of air forced through the stack by a compressor. Some of the oxygen is consumed by the stack and some of the oxygen is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
In fuel cells and fuel cell stacks, it is necessary, upon assembly, for the fuel cell stack to be compressed. The compression load is typically carried by diffusion media (DM) of the UEA in an active area of the fuel cell plate to reduce the electrical contact resistance between the DM and the fuel cell plates. To ensure that the DM carry the compression load, clearance gaps between the bipolar plate assemblies and membranes or shims/subgaskets in the non-active feed regions are required. Due to variations in tolerances in component thicknesses, the clearance gaps in the non-active feed regions may vary between fuel cells, thereby producing variations in the heights of the reactant channels whereby reactant flow may be affected. Furthermore, the clearance gap may allow the UEA to shift from side to side between adjacent fuel cell plates due to differences in reactant pressures, thereby further effecting reactant flow. To control clearance gaps to provide reactant flow uniformity and pressure within the fuel cell stack, the coolant flow is caused to flow at a pressure above the pressure of the reactant flow in an amount sufficient to expand the feed regions of adjacent fuel cell plates to contact the UEA disposed therebetween to militate against the movement thereof.
It would be desirable to develop a fuel cell assembly with an improved means for properly aligning adjacent fuel cell plates and a UEA disposed therebetween to maximize the reactant flow across the fuel cell plates.